This invention relates to signaling congestion control in connection-oriented networks and more particularly to controlling network congestion due to a high number of network-attached devices demanding simultaneously access to a common network-attached resource.
Modern data terminal equipments (DTEs) attached to connection-oriented networks have a very high processing capacity and are consequently very demanding in terms of connections to the network. In order to establish a connection, a data terminal equipment must exchange a signaling protocol message with the network, commonly called xe2x80x9ccall setup messagexe2x80x9d. Network access switching nodes can support a large number of attached-to devices and therefore may process hundreds of call setup messages simultaneously. In large networks nodes may even process thousands of such call setup messages in a very short time.
In some situations, a burst of simultaneous call setup messages may flow into a network causing congestion. For example, when a common resource such as a file server goes down all attached-to devices will attempt to reconnect simultaneously. This congestion may take place either at a network input access node that supports numerous devices requesting the common resource or at the output access node where the common resource attaches to the network.
Such a situation arises in a LAN (local area network) environment when based on a protocol such as ATM (asynchronous transfer mode). Most of the existing ATM LANs rely on the emulation of well-known higher layer LAN protocols such as Ethernet, Token-Ring or Internet Protocol (IP), thereby creating a virtual LAN over the ATM layer. This LAN emulation is enabled by using dedicated protocols, the most widespread being the so-called xe2x80x9cClassical IP over ATMxe2x80x9d protocol and the so-called xe2x80x9cLAN Emulation over ATMxe2x80x9d protocol. In each case, a protocol server is required to manage the virtual LAN over the ATM layer, and consequently, any terminal device that wants to enter the virtual LAN must connect to this protocol server prior to proceeding with any other activity such as data transmission. Thus, signaling congestion may occur when too many data terminal equipments (DTEs) try to connect simultaneously to the protocol server.
Several approaches can be used to address this type of signaling congestion problem. One approach consists in xe2x80x9cjust doing nothingxe2x80x9d, that is, let the network recover from congestion by itself. When bursts of call setup messages are received, many of them are rejected. The rejected devices will retry to connect, and hopefully the connection requests will desynchronize with over time so as not to create a congestion state again. Unfortunately, there is no guarantee that the connection requests will desynchronize. Also the time interval for the requests to desynchronize is not determinable. Furthermore, this approach is not scalable in that if more devices share the same resource, the congestion will worsen. Therefore, this approach is not satisfactory in the context of a high speed/performance network.
Another approach is to increase the processing power of the switching nodes. This would be acceptable if networks were static and not continuing to increase in size and utilization. Networks grow faster than the processing power of the switching nodes which makes this approach a short term solution. Therefore, unsatisfactory.
Still another approach is to implement random timers in the terminal devices to manage the retry procedure. The random timers can induce the desynchronizing of all the source devices requesting connections, and therefore, they can naturally pace the call Setup messages. This approach is similar to the so-called Ethernet Backoff Timer method which is commonly implemented to solve access collision problems. However, it has the disadvantage that it depends on changes to the devices that connect to the network. This makes implementing difficult given multi-vendor multiproduct nature of most devices attaching to a network. Furthermore, no standard appears to exists that requires the terminal devices to implement such mechanisms. Finally, it is not desirable to rely on the behavior of unknown devices to protect a switching node and the network from call setup congestion.
Yet a further approach consists in limiting the number of call setup messages in the switching node in order to protect it against an overflow of such call setup messages. This solution is not very efficient since it induces a random discarding of the pending call setup requests. This can be prejudical for example, when, as it may happen, a group of connections has to be established in such a way that, if only one connection from the group fails, then all the group is torn down and the whole group must be re-established. For instance, this is the case for the control connections in LAN Emulation. Furthermore, this technique is not fair as all the users are penalized while only a few of them may have caused the congestion.
Therefore, there is a need for a solution to the above problems of the prior art that provides efficient protection to network devices from call setup overflow, while assuring the scalability of the networks. Such a solution is provided by the present invention as described hereinafter.
An object of the present invention is to provide a method for keeping control of concurrent connections setup requests in a switching node of a connection-oriented network to provide efficient protection against signaling congestion.
Another object of the invention is to provide a switching node of a connection-oriented network with a system to protect efficiently against call setup message overflow.
In accordance with the appended set of claims, these objects are achieved by providing a method and a system to prevent signaling congestion in a connection-oriented network node in situations where a plurality of network-attached data terminal equipments (source DTEs or source devices) concurrently request a connection to at least one network-attached data terminal equipment (destination DTE or destination device), each of the source DTEs sending call setup messages (CSMs) through the network node to the at least one destination DTE. The CSMs are processed by the network to establish the request connections. The method comprises the steps of: predefining a threshold number (Max) as the maximum number allowed of CSMs from the source DTEs that are actually being processed by the network at a given instant, and predefining a time frame (Window) as the time frame within which no more than Max CSMs are accepted by the network for being processed; detecting each new incoming CSM in the network node; rejecting each new incoming CSM if a number of CSMs equal to Max are already being processed by the network, or if less than Max CSMs are actually being processed by the network, while Max CSMs have already been accepted to be processed during current Window, accepting each new incoming CSM otherwise. The step of detecting each new incoming CSM is optionally followed by a further step of filtering each new incoming CSM to determine whether the CSM satisfies or not at least one predefined filtering criterion, and accepting the incoming CSM if it does not satisfy any of said at least one predefined criterion, or proceeding further with the following steps of the method otherwise.